Mary L. Schapiro, who overhauled the Securities and Exchange Commission after the financial crisis, announced Monday that she is stepping down as chairwoman of the agency.

In recent days, the SEC informed the White House and Treasury Department that Schapiro planned to leave Dec. 14, becoming the first major departure from the Obama administration’s team of financial regulators. Schapiro will also relinquish her position as one of the five members of the agency’s commission, the group that oversees Wall Street and the broader financial markets.

The White House announced Monday that President Barack Obama was naming Elisse B. Walter, a commissioner at the SEC, as the new chairwoman. In a somewhat surprising step, Walter will not be an interim chairwoman but could have the top job for the foreseeable future. Her appointment will not require congressional approval because the Senate previously confirmed her as a commissioner.

Eventually, according to a person briefed on the matter, the White House is expected to nominate another agency chief.

''It has been an incredibly rewarding experience to work with so many dedicated SEC staff who strive every day to protect investors and ensure our markets operate with integrity,’’ Schapiro said in a statement. ‘‘Over the past four years we have brought a record number of enforcement actions, engaged in one of the busiest rulemaking periods, and gained greater authority from Congress to better fulfill our mission.’’

The move, which follows a bruising, four-year tenure, was widely telegraphed. Schapiro, 57, has confided in staff members for more than a year that she was exhausted and hoped to leave after the November elections.

In 2008, Obama nominated Schapiro, a political independent, to head the SEC at a time when extreme economic turmoil had shaken investor confidence in the country’s securities regulators.

The agency was faulted for its lax oversight of brokerage firms like Lehman Brothers, which failed in 2008 and contributed to the worst economic downturn since the Great Depression. Just weeks before Schapiro started as chairwoman, the Wall Street investor Bernard L. Madoff was accused of running a large Ponzi scheme, further damaging the credibility of regulators like the SEC, which missed crucial warning signs about the fraud.

''When Mary agreed to serve nearly four years ago, she was fully aware of the difficulties facing the SEC and our economy as a whole,’’ Obama said in a statement. ‘‘But she accepted the challenge, and today, the SEC is stronger and our financial system is safer and better able to serve the American people — thanks in large part to Mary’s hard work.’’

Schapiro, a lifelong regulator who previously ran the Commodity Futures Trading Commission and the Financial Industry Regulatory Authority, quickly gained a reputation as a consensus builder determined to repair the agency’s reputation. A tireless preparer and self-described pragmatist, Schapiro overhauled the agency’s management ranks, revived the enforcement unit and secured more money and technology at a time when other agencies were being asked to cut back. She also helped craft new rules for Wall Street oversight, as part of the Dodd-Frank regulatory overhaul.

''The SEC came back from the brink,’’ said Harvey L. Pitt, a former chairman of the agency under President George W. Bush. ‘‘I give her enormous credit for that.’’

Consumer advocates and other critics, however, say she failed to grab the bully pulpit at a time the country needed a vocal critic of Wall Street. Since the financial crisis, the agency brought few enforcement cases against the Wall Street executives at the center of the crisis.

The SEC notes it has brought a record number of cases over the last two years. While no top banking executives have been charged, the agency has filed actions against 129 people and firms tied to the crisis.

While Walter will take over as chairwoman, she may not serve the whole term. Mary J. Miller, a senior Treasury Department official, is under consideration for the job, a person briefed on the matter said. Sallie L. Krawcheck, a former top executive at Citigroup and Bank of America, is also in the running, according to people with knowledge of the matter. The agency’s enforcement chief, Robert Khuzami, is a long-shot contender.

As for Schapiro, few expect her to follow her predecessors and move into private legal practice, where she would defend the banks she has spent years regulating. Instead, they say she is more likely to seek out a position at a university or research group.